The floating unit used for the research and exploitation of underwater hydrocarbons require the use of anchorage lines whose extremity is integral with a sea anchor or fixed to a pile, so as to keep these lines in place and stop forces causing them to drift, for example, forces provoked by wind or oceanic currents.
The need to exploit oil deposits at increasingly larger and larger depths requires the use of extremely long lines joining the bottom to the surface.
As regards the anchorage of oil platforms, buoys or storage oil tanks, it may be necessary to install anchorage lines to extremely large water depths of, for example, about from 300 to 1000 meters.
It is common practice to produce an anchorage line in the form of a chain, for example, with welded chain-links. However, in the case of a very long line, for example, a line longer than 3000 meters, the weight of the chain is too heavy in relation to the haulage capacity of this chain.
In effect, a large part of the solidity of the chain is then devolved upon supporting the chain itself and not upon holding the floating vehicle.
Producing an anchorage line in the form of a cable is more appropriate for extremely long anchorage lines due to the cable's high capacity of resistance to traction and a linear weight (with equal resistance) being less than the chain. However, where extremely large lengths are required, it is difficult and often impossible to produce and use a cable as an anchorage line. In particular, the storage and handling of very large lengths of large diameter cables causes problems which are very difficult to resolve.
It has been proposed to use tubular elements with closed extremities and interconnected in an articulated way so as to constitute very long anchorage lines. Each of the essential parts or constituent elements of the line, which is closed at its extremities or ends, so as to be watertight, delimits an interior volume full of air and thus constitutes a float capable of counterbalancing, via the buoyancy in the water, all or part of the weight of the element. Up until now, such anchorage lines have been used as taut lines, namely lines which are in a virtually rectilinear position when in use.
The horizontal pull-back forces or restoring forces of the floating vehicle, when this vehicle tends to go adrift when exposed to the wind and under the effect of oceanic currents, are due to either solely to the elasticity of the line, or to a traction device connected to the anchorage line end situated opposite in relation to the drift or leeway of the vehicle.
When such taut anchorage lines are used, which operate in the same way as bracings, it is obviously advantageous to reduce the apparent weight of the anchorage line when the anchorage line is submerged in the water. It can be particularly advantageous to calculate the essential parts of the anchorage line so as to reduce its apparent weight in the water to approximately nil.
In the case of catenary anchorage lines, namely, anchorage lines assuming a small chain curved form when in service, the problem is completely different, as the horizontal restoring force of the floating vehicle corresponds to the horizontal projection of the line stress at the point linking the anchorage line and the floating vehicle. The stress in the anchorage line depends on the apparent weight of the anchorage line in the water when the horizontal component of the stress at the anchorage point is particularly large in relation to the vertical component when the anchorage line is closer to horizontal at this point.
The lightened anchorage lines known from the prior technique would not make it possible to obtain significant horizontal restoring forces and would not be able to produce highly efficient catenary anchorage lines.
However, in the case of long anchorage lines, lightening under the effect of buoyancy is a considerable advantage to the extent that the actual weight of the entire anchorage line can be significantly reduced.